The present invention relates to an apparatus and a method, for recording information on an optical information recording medium and for reproducing the information from the optical information recording medium, with applying holography therein.
At present, in accordance with a Blu-ray Disc (™) standard, applying a blue-violet semiconductor laser therein enables to commercialization of an optical disc, having a recording density/capacity of around 50 GB even for a consumer product. In the future, such a requirement will be made of enlarging a scale of the capacity, even for the optical disc, to have a capacity as large, as a HDD (i.e., a Hard Disk Drive) has, such as, 100 GB to 1 TB, for example.
However, for the purpose of accomplishing such ultra-high density with an optical disc, there is necessity of a technology, for achieving the high-density by means of a new method, differing from the technology for achieving by means of the short wavelength and the high-NA.
While many studies are made relating to a storage technology for the next generation, attentions are paid upon a hologram recording technology, for recording digital information with using the holography therein.
The hologram technology is a technology for recording information on a recording medium, by placing a signal beam having information of page data, being modulated two-dimensionally by a spatial light modulator, upon a reference light, within the recording medium, and thereby producing a modulation of refractive index within the recording medium through an interference pattern generated at that time.
When reproducing the information, the reference light, which was used when recording, is irradiated upon the recording medium, and this brings about a diffracted light, because the hologram recorded within the recording medium functions, as if a diffraction grating therein. This diffracted light is reproduced as the same light beams; including the signal beam recorded and phase information thereof.
The signal beam reproduced is detected two-dimensionally at high speed, with using an optical detector, such as, a CMOS or a CCD, etc. In this manner, the hologram recording technology enables to record two-dimensional information on an optical recording medium, at once, through one (1) piece of the hologram, and also to reproduce this information, and because it enables overwriting of plural numbers of page data at a certain place on the recording medium; therefore, it enables to achieve recording/reproducing of a large amount of information at high speed.
As one of the hologram recording methods, an angel multiplexing recording is already known. This is a method for conducting a multiplex recording while changing an incident angle of the reference light upon an optical information recording medium. In the hologram recording, efficiency thereof comes up to the best when the signal beam and the reference light are irradiated within the same region, and this results a prevention of unnecessary exposure of the medium, and thereby achieving a high density recording. However, in the angle multiplexing recording, an area changes on the medium where the irradiation is made, because of change of an effective diameter of the light beam when scanning an incident angle of the reference light on the medium. With this, the medium is exposed to the light even in a region, which does not contribute to the recording, and then there is a problem that the recording capacity is reduced down.
As an optical information recording/reproducing technology for dissolving such problem is disclosed, for example, in Japanese Patent Laying-Open No. 2006-17898 (2006) (Patent Document 1). In this publication, it is described that “a laser light emitting from a laser light source is divided into a signal beam and a reference light in a beam splitter, when recording, for keeping a range of irradiation of this reference light upon a hologram recording medium always constant, even if the incident angle of the reference light changes, when recording the hologram in accordance with the angel multiplexing recoding method, and the reference light comes to be parallel lights through a zoom-type beam expander. In that instance, a controller apparatus changes a magnifying power of the zoom-type beam expander depending on a rotation angle of a variable-type rotation mirror, so as to change the diameter of the reference beam, and thereby brining the range of irradiation of the reference light upon the recording medium to be constant, irrespective of the incident angle of the reference light upon the recording medium”.
Also, for example, in Japanese Patent Laying-Open No. 2006-23445 (2006), it is described that “when changing an angle of a scan mirror 12 for chaining an incident angle of a reference light 200 upon a hologram recording material 15, also a slit 11 changes an angle thereof linking with, and thereby changing a beam diameter of the reference light 200 by the slit 11, so that the irradiation range of the reference light 200 upon the hologram recording material 15 comes to be constant, but not changing depending on change of the incident angel of the reference light 200. With this, it is possible to keep the area, upon which this reference light irradiates the hologram material 15, always constant, even if the incident angle of the reference light changes, when recording the hologram in accordance with the angle multiplexing recording method.”